The invention relates to machine elements, such as bearings and gears.
A range of plastics are used as material for machine elements. In addition to good mechanical properties, such as good rigidity and toughness, materials of this type are required above all to have good tribological properties and a good to very good chemicals resistance. Moreover, the materials have to prove themselves under hard conditions and are often exposed to long-term use temperatures of over 140xc2x0 C.
The group of plastics which are used for machine elements under demanding conditions includes thermosets, such as epoxy resins, phenol formaldehyde resins and polyimide resins, and high-temperature thermoplastics, such as polyphenylene sulfide, polyether ether ketone, polyetherimide, polyethersulfone, polysulfone and liquid-crystal polymers, and also industrial thermoplastics, such as polyamide and polyoxymethylene. However, in the case of the latter class of materials, the range of applications is limited with regard to the maximum temperature: Tmax less than 125xc2x0 C. The high-temperature thermoplastics used in machine elements are predominantly reinforced with glass or carbon fibers or mixed with mineral fillers, but are also used in unreinforced or unfilled form. Especially for bearings applications, these thermoplastics are often also modified with graphite, molybdenum disulfide or polytetrafluoroethylene in order to improve the tribological properties (cf. in this respect Gunter Erhard and Erich Strickle, xe2x80x9cMaschinenelemente aus thermoplastischen Kunststoffenxe2x80x94Lager und Antriebselementexe2x80x9d [Machine elements made from thermoplasticsxe2x80x94bearings and drive elements], 2nd Edition, VDI-Verlag GmbH, Dxc3xcsseldorf 1985, pp. 7 to 9).
Compared to metallic materials, unreinforced or unfilled thermoplastics have a thermal expansion which is approximately 10 times greater, generally a lower strength and correspondingly a low load-bearing capacity and a lower modulus of elasticity and a limited high-temperature dimensional stability. Therefore, corresponding machine elements can only be used at relatively low temperatures and under relatively low loads. The profile of properties can be improved by the addition of reinforcing agents and fillers.
When used in machine elements, in addition to the rigidity the flow properties and the resistance to abrasion and also the dimensional stability of the materials play a crucial role. However, particle contamination has proven to be a problem with all materials, for example for the production of bearings and gears, as have the abrasion characteristics in operation. Furthermore, when the machine elements are produced by injection molding, glass fibers are in some cases forced to the component surface or project out of the latter. During subsequent working and/or assembly steps, the protruding glass fibers then break off and may be deposited on the surface of the machine elements, in which case they intensify the friction and wear. Furthermore, the orientation and position of the fibers with respect to the direction of load and movement are crucial to the tribological characteristics especially in the case of bearings. An optimum abrasion and wear resistance is only achieved when the fibers lie parallel to the direction of load and movement. However, particularly in the case of complicated components, this cannot be ensured and areas in which the fibers lie perpendicular to the direction of load and movement are formed; the result is a much worse tribological behavior. Furthermore, the introduction of fibers results in a strongly anisotropic behavior in the material during processing, and this manifests itself during the production of components and samples in different shrinkage characteristics andxe2x80x94as a corollaryxe2x80x94in distortion of the parts. The reason for this is the orientation of the fibers during the filling operation. Specifically, the fibers are oriented in the direction of flow of the molten material, with the result that, in the finished part, the shrinkage is considerably reduced in the direction of the fibers compared to the direction which is transverse to the fibers. To moderate this anisotropic characteristic, fiber/mineral mixtures are often used as a reinforcing system. The thermoplastics often also have to be processed into composite elements, in which the plastic parts are combined, for example, with a steel shell or a steel ring for support. The plastic surface then has to be remachined and lubricated at considerable cost.
It is an object of the invention to design machine elements in such a manner that they are stable and capable of withstanding high loads, the materials used for these elements having favorable tribological properties and good dimensional stability.
According to the invention, this is achieved by the fact that the machine elements consist of a thermoplastic-based glass/plastic compound which contains
a low-melting sulfophosphate glass of the following composition (in mol %): 4 to 10% of Li2O, 4 to 10% of Na2O, 4 to 8% of K2O, 1 to 2% of CaO, 35 to 37% of ZnO, 0 to 3% of La2O3, 19 to 22% of P2O5 and 19 to 22% of SO3, and
a high-performance thermoplastic.
The use of the special glass/plastic compound comprising a low-melting sulfophosphate glass and a high-performance thermoplastic results in machine elements which have a long service life, in particular at elevated temperatures. Moreover, the novel materials have good mechanical properties, such as a high rigidity, dimensional stability and ability to withstand compressive loads. Therefore, the corresponding machine elements also no longer have to be supported with complex metal inlays. A xe2x80x9clow-meltingxe2x80x9d sulfophosphate glass is understood as meaning a glass with a low glass transition temperature Tg, in particular a glass with Tg  less than approx. 500xc2x0 C. A xe2x80x9chigh-performance thermoplasticxe2x80x9d is a high-performance polymer, specifically, in the present instance, a heat-resistant polymer (high-temperature resistant polymer). This is important because both the temperature at which the compound is produced and the processing temperature (of the compound) are  greater than 300xc2x0 C.
The sulfophosphate glass which is contained in the glass/plastic compound has a glass transition temperature in the range from 250 to 280xc2x0 C.; therefore, at the processing temperature it is in the free-flowing state. The compound preferably includes a sulfophosphate glass of the following composition (in mol %): 4.9% of Li2O, 9.4% of Na2O, 7.1% of K2O, 1.6% of CaO, 36.6% of ZnO, 20.0% of P2O5 and 20.4% of SO3. A glass of this type has a glass transition temperature of 268xc2x0 C. Another glass has, by way of example, the following composition (in mol %): 9% of Li2O, 5% of Na2O, 7% of K2O, 1.6% of CaO, 37% of ZnO, 20.4% of P2O5 and 20% of SO3 (Tg=280xc2x0C.). By way of example, a further glass has the following composition (in mol %): 4.8% of Li2O, 9.2% of Na2O, 6.9% of K2O, 1.6% of CaO, 35.9% of ZnO, 2.0% of La2O3, 19.6% of P2O5 and 20.0% of SO3 (Tg =275xc2x0 C.).
As high-performance thermoplastic, the glass/plastic compound preferably contains one of the following polymers: a polyether ether ketone (PEEK), a polyetherimide (PEI), a polyphenylene sulfide (PPS), a partially aromatic polyamide, such as polypthalamide (PPA) and polyamide (PA) 6/6T, or a liquid-crystal polymer (LCP). In these polymers, the glass transition temperature of the glass component is matched to the processing temperature of the thermoplastic material.
Further high-performance thermoplastics which can be used are polyaryl ether ketones (PAEK) in general, for example polyether ketones (PEK), and polysulfones (PSU), in particular polyethersulfones (PES) and polyphenylene sulfones (PPSU).
The proportion of the glass component, i.e. the sulfophosphate glass, in the compound is generally 15 to 80% by weight, preferably 25 to 60% by weight. The processing temperature of the compound is approximately 320 to 420xc2x0 C. Despite the high proportion of glass, the compound has a high ability to flow or very good flow properties.
The glass/plastic compound is advantageously produced, for example, by first of all producing a masterbatch with a glass content of 60 to 90% by weightxe2x80x94at elevated temperature (preferably approximately 320 to 420xc2x0 C.)xe2x80x94from the two components, i.e. sulfophosphate glass and high-performance thermoplastic. Surprisingly, it has been found that, when glass particles (glass grains) with a diameter of xe2x89xa61.5 mm are used in the masterbatch, glass structures in the xcexcm and sub-xcexcm range are obtained, with a uniform distribution.
The further processing then takes place in such a manner that, a result of the addition of further high-performance thermoplastic to the masterbatchxe2x80x94at elevated temperature (preferably approximately 320 to 420xc2x0 C.)xe2x80x94the glass content is reduced, for example, to 25 to 60% by weight. The structure and homogenous distribution of the glass particles are not affected, i.e. are retained. Surprisingly, control experiments have shown that the structure size and distribution of the type described are not obtained if the process proceeds directly from a batch with a glass content of, for example, 15%. Rather, uniformly distributed glass structures, even in the nm range, can only be achieved starting from a masterbatch with a high proportion of the special sulfophosphate glass in a high-performance thermoplastic. As has already been stated, the compound is in the free-flowing state at the processing temperature. The melting process which takes place during the processing means that a new, virgin glass surface is always produced. Therefore, a very smooth surface is obtained and the bonding of the glass particles to the plastic matrix is very good. Compared to conventional fiber-reinforced compounds, there are no exposed glass fibers or glass-fiber ends, and the formation of particles through abrasion is drastically reduced.
Machine elements made from the novel materials, which have good tribological and good shrinkage properties, are in particular bearings, such as sliding-contact bearings, and gears. Examples of further possible uses for these materials are rolls, pulleys, couplings, joints, hinges and spring elements.
The invention is to be explained in even more detail with reference to exemplary embodiments. The sulfophosphate glass used in the tests has the following composition (in mol %): 4.9% of Li2O, 9.4% of Na2O, 7.1% of K2O, 1.6% of CaO, 36.6% of ZnO, 20.0% of P2O5 and 20.4% of SO3.